1. Field of the Invention
The present invention relates to a developing device having a developing roller for bearing and conveying a two-component developer and a layer-thickness regulating member for regulating a layer-thickness of the two-component developer which the developing roller conveys, and to an image forming apparatus having the same.
2. Description of the Related Art
In an electrophotographic image forming process, an electrostatic latent image bearing body uniformly charged is exposed to light corresponding to image information so that an electrostatic latent image is formed on the electrostatic latent image bearing body, and the electrostatic latent image is visualized by a developing device. In order to develop the electrostatic latent image, there have been employed one-component developing method using a magnetic or nonmagnetic one-component developer and a two-component developing method using a two-component developer composed of a toner and a carrier.
In the two-component developing method, a magnetic particle referred to as a carrier and a toner are agitated so that both are charged by a mutual friction therebetween. In this way, the toner is borne on a surface of the carrier. On a surface of a developing sleeve where a magnet is housed internally, the carrier having borne the toner forms into a projection referred to as an ear. The toner included in the ear moves from the developing sleeve to the electrostatic latent image on the electrostatic latent image bearing body, whereby the electrostatic latent image is developed (for example, refer to Japanese Examined Patent Publication JP-B2 3305138).
A device used in the two-component developing method is slightly more complicated than that used in the one-component developing method. However, the two-component developing method is commonly used because of its relative easiness in setting an electric potential of toner and excellence in high-speed responsiveness and stability.
In a device for developing by the two-component developing method where a developing sleeve conveys the developer upwardly in a vertical direction, there exists a problem that a difference in image density occurs between anterior end and posterior end of paper in a print direction.
In order to resolve the problem mentioned above, an art has been disclosed by Japanese Unexamined Patent Publication JP-A 2002-148942, for example. The developing device described in JP-A 2002-148942 is so designed as to restrain the developer having been used for development and stripped from the surface of the developing sleeve from being immediately fed to the developing sleeve and used for development. By doing so, the developing device described by JP-A 2002-148942 prevents density of an image to be formed from declining, thus making it possible to obtain an image having a sufficient image density.
A magnet member having a plurality of magnetic poles is fixedly disposed on an internal side of a cylindrical developing sleeve, a developer composed of a toner and a magnetic carrier is fed to the surface of the developing sleeve by a feeding member, and a development is carried out by rotating the developing sleeve to convey the developer to a developing region facing an image bearing body. The developer having been used for development is sent back into the main body of the device, and is stripped off from the surface of the developing sleeve by a pair of adjacent stripping poles which have the same polarity and are disposed on the above-described magnet member.
The developing sleeve is so rotated that the developer having been used for development is conveyed downwardly in the main body of the device, and an upstream pole having different polarity is so disposed in the magnet member as to be located upstream of the pair of the stripping poles in the rotation direction of the developing sleeve. When assuming a peak value to be A (mT) of perpendicular magnetic force of the upstream pole, assuming a peak value to be B (mT) of perpendicular magnetic force of the first stripping pole located upstream of the rotation direction of the developing sleeve, assuming an angle to be θ1 degree(s) formed among a position a1 where the perpendicular magnetic force of the upstream pole is maximized, a center o of the aforementioned magnet member, and a position a2 where a perpendicular magnetic force between the upstream pole and the first stripping pole is 0 mT, and assuming an angle to be θ2 degree(s) formed among the aforementioned position a2 where the perpendicular magnetic force is 0 mT, the center o of the magnet member, and a position a3 where the aforementioned first stripping pole is maximized, the following formula is satisfied:(A+B)/(θ1+θ2)≧3.02.5>(A/θ1)/(B/θ2)>1.0
However, in the developing device described by JP-A 2002-148942, the developing sleeve has a larger amount of developer in its middle portion than its two end portions, thereby causing a problem that density irregularity occurs in a direction perpendicular to a print direction. Further, there exists a problem that deterioration in image quality ultimately caused by the adherence of the carrier to the photoreceptor occurs owing to the carrier on printing paper.